The mechanisms of synaptic transmission in the vertebrate retina and its role in visual information processing will be studied by intracellular recording from single neurons in the distal retina of the mudpuppy. Different suspected neurotransmitter substances, specific antagonists to these substances, and ions which block transmitter release from presynaptic terminals will be applied to the isolated retina by a perfusion technique. The effects of these substances on membrane potential, membrane resistance, and response to light will be determined in horizontal cells and bipolar cells. It is hoped to determine which substances affect the membrane potential of all postsynaptic cells in a manner similar to the natural transmitter hypothesized to be released by photoreceptors in darkness.